Conventionally, in a lithographic process to produce electronic devices such as a semiconductor (integrated circuit) or a liquid crystal display device, the reduction projection exposure apparatus based on a step-and-repeat method (the so-called stepper) that transfers an image of a pattern of a mask (or a reticle) via a projection optical system onto each of a plurality of shot areas on a sensitive object such as a wafer, a glass plate or the like (hereinafter generally referred to as a “wafer”), on which a resist (sensitive agent) is coated, the projection exposure apparatus based on a step-and-scan method by (the so-called scanning stepper (also called a scanner) and the like are mainly used.
With these types of projection exposure apparatus, a higher resolving power (resolution) is required year by year due to finer patterns according to higher integration of the integrated circuits, and in recent years, exposure apparatus that use the liquid immersion method are beginning to gather attention. As an exposure apparatus that uses the liquid immersion method, an exposure apparatus that performs exposure in a state where the space between the lower surface of a projection optical system and the surface of a wafer is filled locally with liquid such as water, an organic solvent or the like is known (e.g. refer to Patent Document 1). In the exposure apparatus according to Patent Document 1, the resolution is improved by the use of the fact that the wavelength of the exposure light in the liquid becomes 1/n of the wavelength in the air (n is the refractive index of the liquid which is normally around 1.2 to 1.6), and the depth of focus is also substantially enlarged n times than in the air.
Now, in the liquid immersion exposure apparatus, the optical properties (e.g. aberration) of the entire optical system including the projection optical system and the liquid (e.g. water) may fluctuate due to a refractive index change of the liquid by irradiation of an exposure light.
Further, transmittance of the liquid to the exposure light may fluctuate by the liquid coming into contact with the atmosphere. For example, the transmittance of the liquid may decrease by TOC (Total Organic Carbon) in the liquid and/or the increase of dissolved oxygen amount in the liquid.
As a consequence, it may become difficult to maintain or to secure the aberration acceptable, the various precisions required and the like in the exposure apparatus.    Patent Document 1: Kokai (Japanese Patent Unexamined Application Publication) No. 6-252022 bulletin